MOS integrated circuits have gained wide acceptance in the electronics industry. Although "MOS" is an acronym for metal oxide semiconductor, conductive polysilicon gate transistors are now more common than metal gate transistors. As used herein, the acronym "MOS" refers generally to conductor oxide semiconductor devices wherein the gate can be metal, conductive polysilicon or any other conductor. The MOS transistors which are utilized in such integrated circuits are characterized by a threshold gate-to-source voltage required for the transistor to turn on and to conduct drain current. The threshold voltage is susceptible to manufacturing process variations and temperature variations. In many cases, threshold voltage variations are tolerable. In other cases, however, such threshold voltage variations are detrimental to circuit performance. For example, when the inputs to an MOS integrated circuit must be compatible with TTL logic or some other logic family, threshold voltages must be maintained within the limits of the logic family. It is known that the threshold voltages of all MOS devices on an integrated circuit tend to be matched and to track each other, since they are subject to the same temperature variations and the same manufacturing process.
In the past, threshold voltages of MOS circuits have been controlled by relatively complex circuitry. For example, an on-chip variable voltage supply can be utilized since the circuit threshold voltage varies with the supply voltage. Prior art techniques have not been satisfactory since the circuitry occupies a significant part of the chip area, requires significant current for operation, and tends to be difficult to stabilize.
It is a general object of the present invention to provide a circuit for controlling the threshold voltage of integrated circuits utilizing MOS transistors.
It is another object of the present invention to provide an MOS integrated circuit having an onboard circuit for controlling the threshold voltages of one or more of the MOS devices on the integrated circuit.
It is a further object of the present invention to provide an MOS threshold control circuit which requires a relatively small current.
It is yet another object of the present invention to provide an MOS threshold control circuit that is simple in construction and requires a relatively small chip area.
It is another object of the present invention to provide an MOS threshold control circuit which maintains stable threshold voltages during temperature variations.